1. Field of the Invention
The present invention relates generally to methods and systems for spatial-shift, wavelength multiplexing of optical signals in an optical communication system. More specifically, the invention relates to methods and systems for routing optical signals through an optical communication system by spatially shifting the signals and dispersing the signals into discrete regions onto an optical plane.
2. Description of the Related Art
With the advent of high-speed wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) systems over the last few years, optical routers in such systems have played an increasingly significant role for routing multi-wavelength, highly dispersible signals through the system. Wavelength multiplexers include various designs which produce different kinds of output signals for particular applications. Generally speaking, the desired passband response of a WDM router is a flat, uniform response over a pre-determined wavelength range surrounded by sharp drop-offs for out-of-band signals. FIG. 1 illustrates the preferred or optimal passband response of a WDM or DWDM wavelength filter as a function of the transmissivity of the filter. As can be seen in FIG. 1, the response is flat at about 0 dB centered about a center wavelength xcex0. Moreover, the response has a sharp transition region to which tends to ensure that good signal transmission is achieved with little loss in the signal.
The fundamental problem associated with most diffraction grating routers is that imaging through a diffraction grating creates a linear shift of the focal spot as a function of wavelength, a result which is conflict with the goal of achieving a segmented, flat-topped passband that is inherently a non-linear function. For example, a collimated beam can illuminate a sequence of dielectric notch filters so that each reflects one wavelength range. This arrangement can produce a flat passband, but since the insertion loss scales linearly with the number of wavelength channels, it is not generally regarded as suitable for large channel count WDM or DWDM systems. Array waveguide routers (AWR), sometimes referred to as xe2x80x9cDragonexe2x80x9d routers, can provide large channel counts of eighty or more but tend to produce Gaussian shaped passbands that do not have sharp transition points and are therefore quite lossy.
Free-space optical wavelength routers have been manufactured using a combination of lenses, gratings and fiber or waveguide input output elements. However, these types of routers include waveguides that have a small width core relative to the waveguide cladding layer and a minimum pitch between guiding channels on a fiber array or multi-waveguide substrate. The lateral alignment tolerance necessary to couple with less than about ten percent loss is typically one to three microns, whereas the pitch between adjacent output waveguides is from twenty in two hundred and fifty microns. This tends to create a narrow passband shape with a broad xe2x80x9cdeadxe2x80x9d region between center wavelengths, which is highly disadvantageous for WDM and DWDM systems. To alleviate this problem, a combination of optical defocus elements, mode-expanding waveguide shapes, and closely spaced output channels can flatten the passband and reduce the dead space; however, these techniques also tend to create excess loss and reduce optical throughput efficiency.
There accordingly exists a long-felt but unresolved need in the art for methods and systems for imaging with optical routers in a communication system which produces a segmented, flat-topped passband. It would be desirable if such methods and systems look advantage of the linear dependent wavelength shift associated with spatial-shift wavelength elements in which wavelength and imaging dispersion determines a region to illuminate with an optical spectrum. It would further be desirable if the spatial shift in the spectrum were defined simply by the surface geometry of the element since this will produce a clean spatial shift with little to no loss of signal. Such needs have not heretofore been met or fulfilled in the art.
The aforementioned long-felt needs are met, and problems solved, by optical routers and methods for routing optical signals through optical communication systems provided in accordance with the present invention. The inventive methods and systems provide a spatially-shifted and multiplexed signal by first linearly dispersing a spectrum comprising a plurality of wavelengths to create a plurality of discrete regions of signal on an intermediate image plane. The linearly dispersed regions are then spatially disbursed and the discrete regions are re-imaged to remove the dispersion associated with linearly dispersing the spectrum.
The optical routers and methods provided in accordance with the invention thus achieve efficient spatial shifting of wavelengths to multiplex signals traversing the communication system. By first linearly dispersing the region into discrete regions and then spatially shifting the regions, a flat passband with sharp transition regions as a function of wavelength can be realized. Moreover, through the use of simple optical conditions, optical routers claimed and described herein are economical and easy to fabricate. Such results have not heretofore been achieved in the art.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.